The proliferation of digitized media such as image, video and multimedia is creating a need for a security system which facilitates the identification of the source of the material.
Content providers, i.e. owners of works in digital data form, have a need to embed signals into video/image/multimedia data which can subsequently be detected by software and/or hardware devices for purposes of authenticating copyright ownership, control and management.
For example, a coded signal might be inserted in data to indicate that the data should not be copied. The embedded signal should preserve the image fidelity, be robust to common signal transformations and resistant to tampering. In addition, consideration must be given to the data rate that can be provided by the system, though current requirements are relatively low--a few bits per frame.
In U.S. patent application Ser. No. 08/534,894, filed Sep. 28, 1995,. entitled "Secure Spread Spectrum Watermarking for Multimedia Data" now abandoned and assigned to the same assignee as the present invention, which is incorporated herein by reference, there was proposed a spread spectrum watermarking method which embedded a watermark signal into perceptually significant regions of an image for the purposes of identifying the content owner and/or possessor. A strength of this approach is that the watermark is very difficult to remove. In fact, this method only allows the watermark to be read if the original image or data is available for comparison. This is because the original spectrum of the watermark is shaped to that of the image through a non-linear multiplicative procedure and this spectral shaping must be removed prior to detection by matched filtering and the watermark is inserted into the N largest spectral coefficients, the ranking of which is not preserved after watermarking. Thus, this method does not allow software and hardware devices to directly read embedded signals.
In an article by Cox et al., entitled "Secured Spectrum Watermarking for Multimedia" available at http://www.neci.nj.com/tr/index.html (Technical Report No. 95-10) spread spectrum watermarking is described which embeds a pseudo-random noise sequence into the digital data for watermarking purposes.
The above prior art watermark extraction methodology requires the original image spectrum be subtracted from the watermark image spectrum. This restricts the use of the method when there is no original image or original image spectrum available. One application where this presents a significant difficulty is for third party device providers desiring to read embedded information for operation or denying operation of such a device.
In U.S. Pat. No. 5,319,735 by R. D. Preuss et al entitled "Embedded Signalling" digital information is encoded to produce a sequence of code symbols. The sequence of code symbols is embedded in an audio signal by generating a corresponding sequence of spread spectrum code signals representing the sequence of code symbols. The frequency components of the code signal being essentially confined to a preselected signaling band lying within the bandwidth of the audio signal and successive segments of the code signal corresponds to successive code symbols in the sequence. The audio signal is continuously frequency analyzed over a frequency band encompassing the signalling band and the code signal is dynamically filtered as a function of the analysis to provide a modified code signal with frequency component levels which are, at each time instant, essentially a preselected proportion of the levels of the audio signal frequency components in corresponding frequency ranges. The modified code signal and the audio signal are combined to provide a composite audio signal in which the digital information is embedded. This component audio signal is then recorded on a recording medium or is otherwise subjected to a transmission channel. Two key elements of this process are the spectral shaping and spectral equalization that occur at the insertion and extraction stages, respectively, thereby allowing the embedded signal to be extracted without access to the unwatermarked original data.
In U.S. patent application Ser. No. 08/708,331, filed Sep. 4, 1996,entitled "A Spread Spectrum Watermark for Embedded Signaling" by Cox; now U.S. Pat. No. 5,848,155 and incorporated herein by reference, there is described a method for extracting a watermark of embedded data from watermarked images or video without using an original or unwatermarked version of the data. This work can be viewed as an extension of the original work of Preuss et al from the audio domain to images and video.
This method of watermarking an image or image data for embedding signaling requires that the DCT (discrete cosine transform) and its inverse of the entire image be computed. There are fast algorithms for computing the DCT in N log N time, where N is the number of pixels in the image. However, for N=512.times.512, the computational requirement is still high, particularly if the encoding and extracting processes must occur at video rates, i.e. 30 frames per second. This method requires approximately 30 times the computation needed for MPEG-II decompression.
One possible way to achieve real-time video watermarking is to only watermark every N.sup.th frame. However, content owners wish to protect each and every video frame. Moreover, if it is known which frames contain embedded signals, it is simple to remove those frames with no noticeable degradation in the video signal.
In U.S. patent application Ser. No. 08/715,953, filed Sep. 19, 1996,entitled "Watermarking of Image Data Using MPEG/SPEG Coefficients" by Cox, and incorporated herein by reference, there is described an alternative method, which is to insert the watermark into n.times.n blocks of the image (subimages) where n&lt;&lt;N. Then the computation cost is ##EQU1##
For N=512.times.512=2.sup.18 and n=8.times.8=2.sup.6, the asymptotic saving is only a factor of 3. However, empirically the cost of computing the DCT over the entire image may be significantly higher when cache, loop unfolding and other efficiency issues are considered. Thus, the practical difference may approach a 30 fold savings. More importantly, if the block size is chosen to be 8.times.8, i.e. the same size as that used for MPEG image compression, then it is possible to tightly couple the watermark insertion and extraction procedures to those of the MPEG compression and decompression algorithms. Considerable computational saving can then be achieved since the most expenses computations relate to the calculation of the DCT and its inverse and these steps are already computed as part of the compression and decompression algorithm. The incremental cost of watermarking is then very small, typically less than 5% of the computational requirements associated with MPEG.
The present invention improves the reliability of the invention described in the 08/715,953 application, now pending by storing watermark information into subimages, and extracting watermark information from subimages, in a manner different from that described earlier.